1. Field of the Invention
The present invention relates to a cathode material and a non-aqueous electrolyte secondary cell using the cathode material an in particular, to improvement of electric characteristic.
2. Description of the Prior Art
Recently, among chargeable cells, a spotlight is put on a so-called lithium ion secondary cell (non-aqueous electrolyte secondary cell) which repeats charge and discharge by movement of lithium ions and electrons between a positive and a negative electrode, as a cell showing a high voltage characteristic in the order of 4 V.
The lithium ion secondary cell normally uses lithium cobalt oxide (Li.sub.x CoO.sub.2, wherein 0.ltoreq.x.ltoreq.1) for the positive electrode and carbon or graphite for the negative electrode. In such a secondary cell, charge and discharge are carried out by lithium ions which are inserted into and taken out from between layers of the layered compound constituting the positive and negative electrodes.
This Li.sub.x CoO.sub.2 used for the positive electrode is an excellent material exhibiting a high voltage characteristic of 4 V or above and having structure which is maintained comparatively stable for the insert and taken out of lithium ions. However, cobalt is not easily available and costs too much. For this, it is necessary to develop a positive electrode (cathode) material to replace Li.sub.x CoO.sub.2 without using cobalt.
As a cathode material not containing cobalt, there can be considered lithium nickel oxide of layered structure (Li.sub.x NiO.sub.2, wherein 0.ltoreq.x.ltoreq.1) and lithium manganate (LiMn.sub.2 O.sub.4) of spinel structure into which or from which lithium ions and electrons are inserted or taken out to obtain Li.sub.x Mn.sub.2 O.sub.4 (0.ltoreq.x.ltoreq.3). There materials are already used in a part of products. Especially, Li.sub.x Mn.sub.2 O.sub.4 attracts a great attention because manganese is rich as a resource compared to cobalt as well as nickel and costs less.
However, this Li.sub.x Mn.sub.2 O.sub.4 has a problem in the electric characteristic.
That is, the value x in Li.sub.x Mn.sub.2 O.sub.4 is logically in the range of 0.ltoreq.x.ltoreq.3 but because of the structure stability, it is considered to be more appropriate to use the range of 0.ltoreq.x.ltoreq.2.
However, when Li.sub.x Mn.sub.2 O.sub.4 (0.ltoreq.x.ltoreq.2) is used for the cathode of the lithium ion secondary cell, the voltage characteristic obtained is shown in FIG. 2. Although in the range of 0.ltoreq.x .ltoreq.1 it is possible to obtain a high voltage characteristic of 4 V or above, the voltage suddenly drops in the vicinity of x=1, and in the range of 1.ltoreq.x.ltoreq.2, a voltage characteristic in the order of only 3 V can be obtained. (Hereinafter, the range enabling to obtain a voltage characteristic of 4 V or above will be referred to as a 4 V region; and the range enabling to obtain only 3 V or so will be referred to as a 3 V region.)
That is, the 4 V region of the Li.sub.x Mn.sub.2 O.sub.4 (0.ltoreq.x.ltoreq.2) is only about 50% of the logical capacity of Li.sub.x CoO.sub.2 (0.ltoreq.x.ltoreq.1). However, it should be noted that the manganate (Mn.sub.2 O.sub.4) of Li.sub.x Mn.sub.2 O.sub.4 of the spinel structure has a more stable skeleton than the Li.sub.x CoO.sub.2 of the layered structure, and enables to take out sufficient lithium ions. Actually, the Li.sub.x Mn.sub.2 O.sub.4 enables to obtain a capacity of about 80% of the case using Li.sub.x CoO.sub.2. However, in order to obtain a capacity competing with Li.sub.x CoO.sub.2, the 4 V region of the Li.sub.x Mn.sub.2 O.sub.4 should be further extended.